1. Field of the Invention
The present invention relates to an automatic decompression device of a small OHC (Overhead Cam) type engine suitable for a motor cycle, a small all terrain vehicle (hereinafter also referred to as ATV), a snow mobile, a small leisure vehicle, a personal watercraft, or the like.
2. Description of the Related Art
Small OHC-type engines (hereinafter also referred to as small engines) are mounted on some of motor cycles, small all terrain vehicles (small ATV), snow mobiles, small leisure vehicles, personal watercrafts, and the like. Some of the small engines are provided with decompression devices for reducing pressure to reduce staring torque when the engines are starting.
In recent years, there has been developed and adopted an automatic decompression device that is automatically activated when the number of revolutions of an engine is below a predetermined number of revolutions. An example of a prior art is Application for Japanese Utility Model Registration. No. Sho. 58-24605 (Publication for Japanese Utility Model. No. Sho. 59-130011) filed by the applicant of the present invention.
In principle, the automatic decompression device is formed such that when the number of revolutions of a cam shaft of the engine is below a predetermined number of revolutions, a xe2x80x9cdecompression lifterxe2x80x9d protrusibly provided inwardly of a cam face for exhaust is protruded, and a locker arm for exhaust is thereby activated to allow an exhaust valve to be slightly xe2x80x9cOpenxe2x80x9d to make an interior of a cylinder opened in atmosphere to cause a pressure in the cylinder to be reduced, thereby reducing starting torque of the engine.
By the way, it is required that the above-described conventional automatic decompression device be activated only at a low velocity of not more than the predetermined number of revolutions (the number of revolutions in an idling state) and be in a non-operating state at a velocity of not less than the predetermined number of revolutions, because the decompression device needs to reduce the starting torque. Accordingly, to make it possible that a decompression control mechanism provided at one end of the cam shaft is reliably activated at a low speed, i.e., reliably performs switching, in operation of the decompression lifter, the mechanism is provided with an arm-shaped change member passing through a center of the cam shaft and substantially as large as a diameter of a cam sprocket to sense a centrifugal force generated by the revolution of the cam shaft with the high sensitivity. For this reason, a dimension (dimension in a longitudinal thereof) of the change member of the decompression control mechanism is increased. Also, to operate the change member when the number of revolutions of the cam shaft exceeds the predetermined number of revolutions for release of the decompressed state, a link member is swingably provided such that it engages with a cam groove provided at an end portion of the change member and swings by the centrifugal force. The link member is biased by a spring to be swung only when the number of revolutions of the cam shaft exceeds the predetermined number of revolutions.
Consequently, the whole decompression control mechanism, and hence a cylinder head portion of the engine become large. Such an engine is unsuitable as the small engine because it is voluminous. Also, the engine is mounted at a limited position because a space around the cylinder head portion of the engine is small when employed as the small engine for the straddle-type vehicle such as the ATV, the motor cycle, or the personal watercraft. Besides, in the conventional automatic decompression device, since the number of parts and the man-hour for assembly thereof are increased because of its complex mechanism, its cost is correspondingly high.
Under the circumstances, an object of the present invention is to provide an automatic decompression device which is compactly designed and can reduce the number of parts.
According to the present invention, there is provided an automatic decompression device of a small OHC-type engine in which a decompression lifter provided inwardly of an exhaust cam face of a cam shaft in a radial direction of the cam shaft such that the decompression lifter is outwardly protrusible or inwardly retractable in the radial direction from the cam face, for operating by rotational operation of an operating shaft inserted in the cam shaft is protruded by a decompression control mechanism when the number of revolutions of the cam shaft is below a predetermined number of revolutions, to allow an exhaust locker arm to be operated, for reducing a pressure in a cylinder when the engine is starting, wherein the decompression control mechanism comprises: a weight member including a pivot portion pivotably attached to the cam shaft at a position apart from a center axis of the cam shaft and having an outer periphery curved along an outer periphery of a cam sprocket and a tip end portion that engages with the operating shaft, to outwardly swing around the pivot portion by rotation of the cam shaft; and a bias member for biasing a portion of the weight member that is apart from the pivot portion toward the center axis of the cam shaft in a swing area of the weight member, wherein the tip end portion of the weight member that engages with the operating shaft is situated on an opposite side of the pivot portion with respect to the center axis of the operating shaft.
According to the automatic decompression device of the small OHC-type engine configured as described above, since a rotational angle of the weight member required for rotating the operating shaft in a desired angle can be made small, a swing area (swing angle )of the weight member can be reduced. Consequently, the decompression control mechanism has a compact outer shape and the cylinder head portion of the small OHC-type engine can be made compact. Therefore, this engine is suitable as the engine of the straddle-type vehicle such as the motor cycle, the ATV, and the personal watercraft. Further, since the number of parts is reduced, the man-hour for assembly can be reduced, and the cost is low.
It is preferable that in the automatic decompression device, a pair of weight members are provided at a periphery of the cam shaft such that the weight members are symmetric with respect to the center axis of the cam shaft and the bias member is placed between the pair of weight members to bias the weight members to be close to each other, thereby biasing the weight members toward the center axis of the cam shaft in the swing area. With this configuration, the weight members can be made smaller and more compact. Also, this configuration is preferable in terms of dynamic balance.
It is preferable that in the automatic decompression device, the cam sprocket is integrally attached to an end portion of the cam shaft, the weight member is pivotally attached to a side face of the cam sprocket, a restricting protrusion is formed on the side face of the cam sprocket for restricting a swing area of the weight member, and an abutment portion that abuts with the protrusion is formed on the weight member.
It is preferable that in the automatic decompression device, the weight members are provided opposite to each other and one of the restricting protrusions that restricts swing of one of the weight members is formed of a portion of the other weight member. This configuration is preferable because the number of parts can be reduced and the device can be made compact.